NAVIGATION.

Pelorus Guardian and Miners' Advocate., Volume 16, Issue 94, 5 December 1902, Page 2

NAVIGATION.

how shipsl^hT their way ACROSS THE OCEAN.

The “so much desired longitude at sea ” is indeed a vastly important thing to a maritime nation like England ; and it is only in comparatively recent years that it has become possible and easy for vessels to be navigated with safety and convenience upon long voyages.

Years ago only the wealthiest of captains owned chronometers. This instrument is now considered prac-

tically indispensable in navigation ; but in those days it was a new invention, very rare and costly.

Every one who has been a passenger on a great Transatlantic liner of to-day knows what an important, imposing personage is the brassbound skipper. A very different person is lie on the deck of his ship from the modest sea-faring man we meet on land, clad for the time being in his shore-going togs. But. the captain’s dignity is not all brass buttons and braid. He has behind him the powerful support of a deep delightful mystery. He, it is who “ takes the sun,” at noon and finds out the ship’s path at sea. And indeed, regarded merely as a scientific experiment, the guiding of the vessel across the unmarked trackless ocean has few equals within the range of human knowledge. It is the purpose of this brief article to explain the manner in which this seeming impossibility is accomplished. We are not able to go sufficiently into detail to enable him who runs to read and navigate a magnificent steamer, but we hope to diminish somewhat that part of the captain’s vast dignity which depends upon his mysterious OPERATIONS WITH THE SEXTANT, itself. It is nothing but an instrument with which we can measure how high up the sun is in the sky. Now everyone knows that the sun slowly climbs the sky in the morning, reaches its greatest height at noon, and then slowly sinks again in the afternoon. The captain simply begins to watch the sun through the sextant shortly before noon and keeps at it until he discovers the sun is just beginning to descend. That is the instant of noon on the ship. The captain quickly glances at the chronometer and calls out ” noon ” to an officer who is near the instrument. And so the error of the chronometer becomes known there and then without any further astronomical calculations whatever. Incidentally also the captain notes with the sextant how high the sun was in the sky at noon. He has in his mysterious “ chart room ” some printed astronomical tables, which tell him in what terrestrial latitude the sun will have precisely that height on that particular day of the year. Thus, the terrestrial latitude becomes known easily enough, and if only the captain could get his longitude as well, he would known just where his ship was that day at poon We have seen that the sextant observations furnish the error of the chronometer according to ship’s time at noon. In other words the captain is in possession of the correct local time in the place where the ship actually is. Now if he also had the correct time at that moment of some well-known place on shore he would know the difference in time between that place, on shore and the ship. But every traveller on land or sea knows that there is a difference of time between different places on the earth. Now the captain’s chronometer is set to correct ‘‘ Greenwich time ” on shore before the ship

! leaves port. His observations having then told him how much this is wrong on that particular day, and in that particular spot where his ship is, he knows just how far ho has travelled east or west of Greenwich, j In other words he knows his “ longitude from Greenwich,” for longitude is nothing more than distance from Greenwich in an east-and I west direction, just as latitude is in ( the north-and-south direction One of the most interesting BITS OF ASTRONOMICAL HISTORY was enacted in this matter of longitude. From what has been said it is clear that the ship’s longitude will be obtained correctly only if the chronometer has kept exact time since the departure of the ship from port.

Even a very .small error of the chronometer will throw out the longitude a great many miles, and \ye can understand readily that it must be difficult in the extreme to construct a mechinical contrivance capable of keeping exact time when subjected to the rolling and pitching of the sea.

It was as recently as the year 1736 that the first instrument capable of keeping anything like exact time at sea was constructed. It was the work of a watchmaker named John Harrison, and is one of t'he few great improvements in matters scientific which the world owes to a desire for winning a money prize.

It appears that in 1714 a committee was formed by the House of Commons, with no less a person than Sir Isaac Newton himself as one of its members, to consider the desirability of offering Governmental encouragement for the invention of some means of finding the longitude at sea.

Finally the British Government offered a reward of £IO,OOO for an instrument which would find the longitude within 60 miles, £15,000, if within 40 miles and £20,000 if with in 30 miles.

Harrison’s chronometer was finished in 1736, but he did not re-

ceive the final payment of his prize until 1764,